Nature | Vol 584 | 20 August 2020 | 387Article
Additive manufacturing of silica aerogels
Shanyu Zhao1,9 ✉, Gilberto Siqueira2,9, Sarka Drdova3,4, David Norris^1 , Christopher Ubert^1 ,
Anne Bonnin^5 , Sandra Galmarini^1 , Michal Ganobjak1,6, Zhengyuan Pan3,7, Samuel Brunner^1 ,
Gustav Nyström2,8, Jing Wang3,4, Matthias M. Koebel^1 & Wim J. Malfait^1 ✉Owing to their ultralow thermal conductivity and open pore structure^1 –^3 , silica
aerogels are widely used in thermal insulation^4 ,^5 , catalysis^6 , physics^7 ,^8 , environmental
remediation^6 ,^9 , optical devices^10 and hypervelocity particle capture^11. Thermal
insulation is by far the largest market for silica aerogels, which are ideal materials
when space is limited. One drawback of silica aerogels is their brittleness. Fibre
reinforcement and binders can be used to overcome this for large-volume
applications in building and industrial insulation^5 ,^12 , but their poor machinability,
combined with the difficulty of precisely casting small objects, limits the
miniaturization potential of silica aerogels. Additive manufacturing provides an
alternative route to miniaturization, but was “considered not feasible for silica
aerogel”^13. Here we present a direct ink writing protocol to create miniaturized silica
aerogel objects from a slurry of silica aerogel powder in a dilute silica nanoparticle
suspension (sol). The inks exhibit shear-thinning behaviour, owing to the high volume
fraction of gel particles. As a result, they flow easily through the nozzle during
printing, but their viscosity increases rapidly after printing, ensuring that the printed
objects retain their shape. After printing, the silica sol is gelled in an ammonia
atmosphere to enable subsequent processing into aerogels. The printed aerogel
objects are pure silica and retain the high specific surface area (751 square metres per
gram) and ultralow thermal conductivity (15.9 milliwatts per metre per kelvin) typical
of silica aerogels. Furthermore, we demonstrate the ease with which functional
nanoparticles can be incorporated. The printed silica aerogel objects can be used for
thermal management, as miniaturized gas pumps and to degrade volatile organic
compounds, illustrating the potential of our protocol.Aerogels are mesoporous sol–gel materials with high specific surface
area (500–1,000 m^2 g−1) and low density (0.001–0.200 g cm−3), and are
classified as superinsulators owing to their ultralow thermal conduc-
tivity (down to 12 mW m−1 K−1). Silica aerogel is by far the most studied
and used type of aerogel. It is available in bulk quantities for industrial
and building insulation^5 ,^12 , with a rapidly growing market of around
US$220 million per year^14. Although aerogels can have exceptionally
high strength-to-weight ratios^2 , silica aerogels are generally brittle and
impossible to machine by subtractive processing. The viability of addi-
tive manufacture of aerogels has been demonstrated for graphene^15 ,^16 ,
graphene oxide^17 ,^18 , carbon nitride^19 , gold^20 , resorcinol-formaldhyde^21 and
cellulose^22 –^24 , but is “fraught with experimental difficulties and probably
not feasible for silica aerogels”^13. Silica particles are a common additive
for 3D printing^25 , but an additive manufacturing protocol for pure silica
aerogel has not been established. A recent study on biopolymer–silica
hybrid aerogels^26 found that they had poor shape fidelity. In addition,
the biopolymer additives were retained in the final product, resulting in
limited temperature stability and high thermal conductivities.
We print pure silica aerogel objects by direct ink writing (Fig. 1a–e,
Extended Data Fig. 1) of a slurry of silica aerogel powder (IC3100,
Cabot; particle size, 4–20 μm; Fig. 1f) in a 1-pentanol-based silica
sol. The low vapour pressure of pentanol (18 times lower than that of
water at 20 °C) prevents drying-induced surface damage, even when
printing for extended durations (Extended Data Table 1). The added
cost of using industrial-grade silica aerogel powder is negligible for
miniaturized applications. The high loading of gel particles (at least
40 vol%) means the ink exhibits the shear thinning behaviour required
for direct ink writing (Fig. 1g, h, Extended Data Fig. 2). The ink con-
sists of aerogel grains roughly 10 μm in diameter suspended in a sol
with silica nanoparticles around 10 nm in diameter. The rheological
behaviour is complex: strain overshoot at large-amplitude shear, as
is typical for colloidal suspensions^27 , and non-Newtonian shear thin-
ning at small-to-medium-amplitude oscillatory shear, as is typical for
suspensions of large particles. The addition of poly(propylene glycol)
bis(2-aminopropyl ether) increases the viscosity of the ink, prevent-
ing solid–liquid phase separation, and improves its homogeneityhttps://doi.org/10.1038/s41586-020-2594-0
Received: 11 November 2019
Accepted: 28 May 2020
Published online: 19 August 2020
Check for updates(^1) Laboratory for Building Energy Materials and Components, Swiss Federal Laboratories for Materials Science and Technology, Empa, Dübendorf, Switzerland. (^2) Cellulose and Wood Materials
Laboratory, Swiss Federal Laboratories for Materials Science and Technology, Empa, Dübendorf, Switzerland.^3 Institute of Environmental Engineering, ETH Zurich, Zürich, Switzerland.
(^4) Laboratory for Advanced Analytical Technologies, Swiss Federal Laboratories for Materials Science and Technology, Empa, Dübendorf, Switzerland. (^5) Swiss Light Source, Paul Scherrer
Institute, Villigen, Switzerland.^6 Faculty of Architecture, Slovak University of Technology in Bratislava, Bratislava, Slovakia.^7 State Key Laboratory of Pulp and Paper Engineering, South China
University of Technology, Guangzhou, China.^8 Department of Health Sciences and Technology, ETH Zurich, Zurich, Switzerland.^9 These authors contributed equally: Shanyu Zhao,
Gilberto Siqueira. ✉e-mail: [email protected]; [email protected]